Manually operated glue guns typically include a tube-shaped heating chamber for receiving a glue stick at its rear end and a nozzle on its front end for discharging melted material. As the solidified glue stick is advanced into the heating chamber the portion of the glue stick which is in the heating chamber is heated to beyond its melting point and the remainder of the still solid glue stick acts as a piston to push the melted glue out of the nozzle and onto the workpiece. The heating apparatus for such devices have previously used a resistor and associated thermostat. Such prior art devices required a relative long time for preheating so that the predetermined operating temperatures would be reached along the channel containing the glue stick. Such prior art devices have not been entirely satisfactory in use and substantial variation occurred in the quality of the joints achieved with the glue.
As a result, it has been proposed to use electrical heating apparatus which include a positive temperature coefficient ("PTC") heating resistor arranged in an axial direction relative to the channel in which the glue stick is introduced.
As is well known, and described for example in U.S. Pat. No. 4,493,972, the use of PTC heating resistors has the advantage of requiring very little power during preheating and during standby. In addition, use of such resistors increase the uniformity of the viscosity of the binding material and allows the thermostat switch of prior art devices to be dispensed with. Typically, conventional PTC resistor cartridges used in glue guns consist of a central resistant element or elements engaged on opposite sides by pressure bodies. The cartridge assembly is held together in various ways, typically for example by an elastic coating as disclosed in U.S. Pat. No. 4,493,972.
PCT resistance cartridges of the prior art are somewhat difficult to assemble. In addition, because the cartridges heat and cool repeatedly, there is substantial expansion and contraction within the housing of the glue gun. However, it is important in order to maintain uniformity of the heat applied to the housing for the cartridge to remain in continuous contact with the housing and previously proposed PTC cartridges could not achieve this desirable result because of their expansion and contraction. Thus uniform heating of the housing is not always accomplished.